Field of the Invention
The invention relates to a method for selecting an interlayer having an acoustic damping property in order to be incorporated into a laminated glazing unit, intended in particular for vehicles of locomotion, in particular a motor vehicle.
Discussion of Background
Among all the qualities contributing to comfort in modern means of transport such as trains and motor vehicles, silence has become the determining factor.
Acoustic comfort has been improved now over several years, by dealing with noise, such as noise from the engine, tires or suspension, and this at their source or during their propagation through the air or solids, by means for example of absorbent coatings or elastomeric connecting components.
The shapes of vehicles have also been modified in order to improve penetration through the air and to reduce turbulence that is itself a source of noise.
For several years, emphasis has been given to the role that glazing units can play in improving acoustic comfort, in particular laminated glazing units comprising plastic interlayer films. Laminated glazing units have, in addition, other advantages such as eliminating the risk of flying fragments in the case of sudden breakage, and impeding break-ins.
It has been demonstrated that the use of standard plastic films in laminated glazing units is not suitable for improving acoustic comfort. Specific plastic films have thus been developed that have damping properties enabling acoustic comfort to be improved.
In the following description, reference to a damping film relates to a viscoelastic plastic film that provides improved damping of vibrations in order to give the glazing unit a function of noise reduction.
It has been shown that the acoustic performance of a glazing unit depends on the value of the loss factor tan δ of the material constituting the interlayer film. The loss factor is the ratio between the energy dissipated in the form of heat and the elastic strain energy; it characterizes the capacity of the material to dissipate energy. The higher the loss factor, the greater the energy dissipated and therefore the more the material plays its damping role.
This loss factor varies as a function of the temperature and of the frequency. For a given frequency, the loss factor reaches its maximum value at a temperature known as the glass transition temperature.
The materials used as interlayers of laminated glazing units are viscoelastic plastic films, of acrylic polymer or acetal resin or else polyurethane type for example, which have quite a high loss factor, such as at least greater than 0.6 for a given temperature range and for a given frequency range.
The loss factor tan δ is assessed using a viscoanalyzer. The viscoanalyzer is a machine that makes it possible to subject a specimen of material to strains under precise temperature and frequency conditions, and thus to obtain and process all of the rheological magnitudes that characterize the material.
It has also been described, regarding the integration of a damping interlayer in a laminated glazing unit, that the loss factor tan δ should not be considered alone but that the shear modulus G′ constitutes another characteristic to be taken into account in the damping property of the interlayer. Document EP-A-844 075 teaches that, in order to damp vibrations, the interlayer of the laminated glazing unit must correspond to particular values as regards the shear modulus G′ and the loss factor tan δ. It is recalled that the shear modulus G′ characterizes the stiffness of the material; the higher G′, the stiffer the material, and the lower G′, the more flexible the material. The shear modulus depends on the temperature and on the frequency. The shear modulus G′ is also assessed using a viscoanalyzer. This document describes that the loss factor tan δ of the interlayer is greater than 0.6 and the shear modulus G′ of the interlayer is less than 2×107 Pa for temperatures between 10° C. and 60° C. and frequencies between 50 Hz and 10 000 Hz, in order to damp, more particularly, noises of solid-borne origin.
Furthermore, when the laminated glazing unit is used as a windshield, it is subjected to acoustic vibrations that are specific to it. Thus, the four first natural frequencies of the windshield, and in particular the second and third natural frequencies of the windshield, between 100 Hz and 240 Hz, are particularly annoying, acoustically speaking. The interlayer of document EP-A-844 075 is suitable for damping solid-borne noises, but not for the vibroacoustic damping of the first natural frequencies of the windshield, in particular the second and third natural frequencies.
There is therefore a need for a method for selecting an interlayer which makes it possible to optimize the damping of the first natural frequencies of a windshield, in particular of the second and third natural frequencies of the windshield, without weighing down the windshield.